Redarc charge equalisers, the CE25 at 25A and the CE40S2 at 40A, which take a regulated 12V line off a 24V system.
On vehicles with a 24V system, lorries and buses, the classic problem is powering 12V equipment: tapping one of the two series batteries works for a few minutes, then that battery discharges more than the other and the imbalance puts both out of service. The equaliser draws from the whole 24V pack and gives back 12V, keeping the two batteries aligned.
Size it on the sum of the 12V loads, with margin for the inrush of pumps and compressors.
